Electric high pressure discharge lamp



Feb. 10, 1942. KERN ETAL 2,272,467

ELECTRIC HIGH PRESSURE DISCHARGE LAMP Filed Sept. 7, 1939 'INVENTORS Josef' K e rn Hermann Kr-efft BY TTORNEY Patented Feb. 10, 1942 2,272,467ELECTRIC HIGH PRESSURE DISCHARGE LAMP Jose! Kern and Herman KreiIt,Berlin-Schoneberg, Germany, assignors to General Electric Company, acorporation of New York Application September .7, 1939, Serial No.293,850 In Germany September 10, 1938 2 Claims. (01. 17s 122) Thisinvention relates to electric high pressure discharge lamps withdischarge vessel of glass with high melting point, especially quartzglass, which contains solid thermionic electrodes, a rare gas fillingand a small quantity of mercury and is so loaded that an operating vaporpressure of many atmosphere is produced.

The object of the invention is, to produce such a high pressuredischarge lamp for power inputs of less than 200 watts, which can beoperated by ordinary mains voltages and is well suited for generalilluminating purposes. At the same time the lamp should not require acomplicated cooling arrangement, a such as the known watercooled highpressure capillary lamps.

In solving this problem the invention is based on a new discovery.Hitherto it was always pointed out, that high pressure discharge lamps,especially those with are burning voltage approximating the mainsvoltage, are extremely sensitive to fluctuations in voltage, because forexample in the event of a sudden increase in the supply voltag thecurrent-limiting series resistance determining the discharge currentintensity must alone. take upthe whole excess voltage. Consequently. a5% increase in supply voltage on a series resistance taking up about 20%of the supply voltage results in a 25% excess voltage which causes anincrease of 25% in the discharge current. It was the general opinion ofthose versed in the art, that in reality the conditions were even worsebecause the are burning voltage not only does not increase as thecurrent intensity increases but even drops, which particularly favorsthe occurrence of excess current.

However, it is particularly important that the power input and th lightradiation of a lamp for general illumination and many other purposesshould remain constant as far as possible. Otherwise the unavoidablevoltage fluctuations occurring in meshed network lead to intolerablefluctuations in light and' hence the lamps would show in adisadvantageous manner very diiierent lighting powers, according to thepoints at which the lamps are connected to the current network subjectto drop in voltage.

Now it has been unexpectedly discovered that by no means all highpressure discharge lamps, as was generally imagined, have a droppingcurrent voltage characteristic curve but on the contrary thecharacteristic curve dropping in the case of small arc powereverycentimeter of the length of the arc becomes less steep as the are powerincreases, then merges into a horizontal section and then into a gentlyrising section and finally, in th case of very high power inputexceeding 300 watts per centimeter of the length of the arc, shows avery considerable rise.

In the new lamp this peak range of the specific power input of the arcis utilized in an advantageous manner for avoiding the above mentionedobjections. A number of additional advantages are attained, for examplethe advantage that, thanks to the steep rise of the characteristiccurve, the series resistance hitherto generally used can be made verymuch smaller and under certain circumstances may even be omitted in theoperation of the lamp.

The using of the peak range of the specific power characteristic curveof the are means further, that the relatively small lamp power 01' 40watts for example on an extremely short arc section is converted intoradiation, that is the electrode spacing for a lamp of a certain poweris chosen extremely small. in turn enables on' the other hand thespherical or approximately spherical vessel used in these lamps to be ofvery small dimensions, less than 15 millimeters and to be consequentlymuch more resistant to pressure, to heat more quickly when the lamp istaken into service, to be produced cheaply from very little material andrendered safe in service.

The new high pressure discharge lamp, constructed according to theinvention for mains voltage operation and small power inputs of lessthan 200 watts, is thus characterized in that when using an electrodespacing of less than 5 millimeters the inner surface of the spherical orapproximately spherical discharge vessel is less than 1 squarecentimeter for every 25 watts power input of the lamp and, in the caseof an operating pressure of more than 50 atmospheres for obtaining aconsiderably steeper current voltage characteristic curve, the specificpower input of the arc exceeds 300 watts per centimeter of the arclength.

Experiments which have been carried out with such extremely smallmaximum pressure lamps have shown that, if the lamp is made carefully,the dreaded wall blackening is avoided to' a great extent and a.sufliciently long lif is attained. Presumably the evaporation andparticularly the atomization of the electrodes is suppressed to anunexpectedly great extent by the exceptionally high operating vaporpressure.

Small spherical high pressure discharge lamps with an internal diameterof about 15 millimeters, an operating vapor pressure exceeding 10atmospheres and a power input exceeding to be easily This short arclength watts per centimeter have been proposed for the production of lowpower lamps, but it has not been recognized that very great andentirelyunexpected advantages are derived if the pressure and power valuesindicated are very considerably exceeded and at the same time the arcand the lamp vessel are confined to extremely small dimensions. Theseadvantages outweigh by far the supposed disadvantage of the presence orexccptionally high operating vapor pressures which were hitherto usedonly for lamps of an entirely diilerent type and only for the productionof maximum illuminating densities.

An embodiment of the invention is illustrated by way of example in theonly figure of the accompanying drawing which shows in natural size ahigh pressure lamp enclosed in an enveloping vessel with socket for apower input of 50 watts.

The spherical discharging vessel I of quartz glass has an internaldiameter of only about 6.5 millimeters, that is an internal surface areaof about 1.3 square centimeter, so that the specific watt load of theinner surface is about 31 watts per square centimeter, that isconsiderably more than 25 watts.

The electrodes 2 are arranged at a mutual distance of about 2millimeters and consist of small hollow tungsten bodies provided withapertures and containing a small quantity of thorium oxid or zirconiumoxide. tion of the glowing electrodes may be dispensed with. Theelectrode bodies 2, as shown in the drawing, are mounted on the wall ofthe "vessel, so that a favorable cooling of the electrodes is attainedowing to the heat being conducted off by the thick walled quartz glassvessel. The current leads 3 of the electrodes 2 are fused with the aidof molybdenum bands 4 into the diametrically opposite, outwardlyprojecting supports 5 which are fixed on the holding wires 1 by means ofcarrier straps 6. These holding wires 1 are electrically connected atone end to the socket contacts 8 of the enveloping vessel 9 and at theother end to the electrodes 2 by the wires I0.

For facilitating the ignition the discharging vessel I has a filling ofrare gas, such as argon, at a pressure or 20 millimeters. For obtaininga. rapid heating of the discharging vessel a fundamental gas filling ofconsiderably higher pressure, for example a pressure of 400 millimeters,

may, however, be used.

Moreover, the discharging vessel I contains a certain amount of mercuryII, the quantity of which is so measured that, when the lamp is inservice, an undersaturated vapor atmosphere at a pressure of about 130atmospheres occurs.

The enveloping vessel 9 has a nitrogen fiilling at a pressure of about300 millimeters. If a par- If desired a separate activaticularlyintensive cooling of the discharging vessel I appears necessary, theenveloping vessel may also contain a gas of high heat conductivity, suchas hydrogen.

The enveloping vessel I is preferably made to diffuse light so as toavoid dazzling. Luminous substances may also be provided or luminousglass used for additionally utilizing the considerable ultravioletradiation of the lamp. Owing to the extremely high specific power inputof the arc the high pressure lamp according to the invention has a veryhigh red content, which renders the lamps particularly suitable forgeneral illumination.

We claim:

1. An improved lamp which is capable of operating without series ballastand comprising the combination of a translucent substantially sphericalvessel of high melting point material and of internal diameter less than15 millimeters, spaced solid electrodes in said vessel and less than 5millimeters apart, mercury in the vessel in such amount as to becompletely vaporized at an operating pressure on the order of onehundred and thirty atmospheres, and a power source connected to thelamp, said source having a potential of such value that energy issupplied in excess of 25 watts for each square centimeter of internalsurface area of the vessel and also in excess of 300 watts for eachcentimeter of the length of the discharge between the electrodes,whereby the lamp operates with an internal pressure in excess of 50atmospheres and exhibits a sharply rising current-voltagecharacteristic.

2. An improved lamp which is capable of operating without series ballastand comprising the combination of a spherical quartz vessel of internaldiameter of about 6.5 millimeters, said vessel containing a fixed gasand mercury in such amount as to be completely vaporized at an operating pressure on the order of one hundred and thirty atmospheres,solid electrodes projecting inwardly from the wall of the vessel atdiametrically opposite points thereof and having their extremities about2 millimeters apart, and a power source connected to the lamp, saidsource having a potential of such value that energy is supplied inexcess of 25 watts for each square centimeter of surface area of thevessel and also in excess of 300 watts for each centimeter of the lengthof the discharge between the electrodes, whereby the lamp operates withan inter nal pressure in excess of 50 atmospheres and exhitbits asharply rising current-voltage character- 1s 1c.

JOSEF KERN. HERMANN KREFF'I.

